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Acids and Bases

23-3-2011

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Definition of Acids

Arrhenius acid: A substance that releases H+ in water ( e.g. HCl)

H+ + H2O H3O+ Hydronium Ion

Acid: A substance which increases [H+] when dissolved in water.

Base: A substance which increases [OH-] when dissolved in water.

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Arrhenius Acids

- Strong acids completely ionize in water to release H+ (H3O+) and an anion;

Strong Acids:HClO4 H2SO4 HI HBr HCl HNO3

HI + H2O H3O+ + I- or HI H+ + I- (100%)

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Arrhenius Acids

- Weak acids only partially ionize in water;

Weak Acids:

HC2H3O2 HF H3PO4 HCN H2S HNO2 NH4+

HC2H3O2 H+ + C2H3O2-

C CHH

HO

OH

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Brønsted-Lowry Acids and Bases

The Arrhenius definition does not apply when water is not the solvent; so, a broader definition (Brønsted-Lowry) was generated.

Acid: A proton donor

Base: A proton acceptor

Example:

HCl(aq) H+(aq) + Cl-

H+(aq) + :NH3(aq) NH4

+(aq)

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Conjugate Acids and Bases:

• From the Latin word conjugare, meaning “to join together.”

• Reactions between acids and bases always yield their conjugate bases and acids.

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Examples: HF + HCO3- H2CO3 + F-

HF = Acid F- = Conjugate Base

HCO3- = Base H2CO3 = Conjugate Acid

CH3NH2 + HI CH3NH3+ + I-

Which is conjugate acid & which is conjugate base?

I- = conjugate base CH3NH3+ = conjugate

acid

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Strong Acids

You will recall that the six strong acids are HCl, HBr, HI, HNO3, H2SO4, and HClO4.

These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.

For the monoprotic strong acids,

[H3O+] = [acid].

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Strong Bases

Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).

Again, these substances dissociate completely in aqueous solution.

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Autoionization of Water

H2O(l) + H2O(l) H3O+(aq) + OH−(aq)

Autoionization is the self-ionization of a neutral

molecule into positive and negative ions.

What is the equilibrium-constant expression for this equilibrium?

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At 25°C, Kw = 1.0 10−14 and [H+] = [OH-] = 10-7

Kw is also called ion-product of water as well as autoprotolysis of water

Kc = [H3O+] [OH-]

Kc is given the special symbol, Kw

The expression is…. 2 H2O H3O+ + OH-

K = [H3O+][OH-]/[H2O]2

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pH Scale

In most cases, the hydrogen ion concentration is very small which makes it difficult to practically express a meaningful concept for such a small value. Currently, the pH scale is used to better have an appreciation of the value of the hydrogen ion concentration where:

pH = - log [H+]pH + pOH = 14

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In the case of pure water at 25oC , the pH is 7 and

the solution is said to be neutral.

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What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 0.013 M?

Kw = [H+][OH-] = 1.0 x 10-14

[H+] = 0.013 M

[OH-] =Kw

[H+]

1 x 10-14

0.013= = 7.7 x 10-13 M

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The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

pH + pOH = 14.00

pOH = -log [OH-] = -log (2.5 x 10-7)

= 6.60

pH = 14.00 – pOH = 14.00 – 6.60 = 7.40

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What is the pH of a 2 x 10-3 M HNO3 solution?

HNO3 is a strong acid – 100% dissociation.

HNO3 (aq) + H2O (l) H3O+ (aq) + NO3- (aq)

pH = -log [H+] = -log [H3O+] = -log(0.002) = 2.7

Start

End

0.002 M

0.002 M 0.002 M0.0 M

0.0 M 0.0 M

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More detailed manipulation:

Find the pH of a 0.1 M HCl solution.

SolutionHCl is a strong acid that completely

dissociates in water, therefore we haveHCl H+ + Cl-

H2O H+ + OH-

[H+]Solution = [H+]from HCl +[H+]from water

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However, [H+]from water = 10-7 in absence of a common ion, therefore it will be much less in presence of HCl and can thus be neglected as compared to 0.1 (0.1>>[H+]from water)

Always neglect [H+]from water when [H+]from

ACID is larger than 10-6M

[H+]solution = [H+]HCl = 0.1pH = -log 0.1 = 1

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Calculate the pH of the solution that is 0.04 M Ca(OH)2 in water.

 

[OH-] = [OH-]Ca(OH)2 + [OH-]water

Always neglect [OH-]water when [OH-]base is larger than 10-6M

[OH-]Ca(OH)2 = 0.08 M 

[H+] = 10-14/0.08 = 1.2x10-12 M, Therefore:

[OH-] = [OH-]Ca(OH)2  = 0.08 MpOH = 1.097pH = 14 – 1.097 = 12.9

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What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution?

Ba(OH)2 is a strong base – 100% dissociation.

Ba(OH)2 (s) Ba2+ (aq) + 2OH- (aq)

Start

End

0.018 M

0.018 M 0.036 M0.0 M

0.0 M 0.0 M

pH = 14.00 – pOH = 14.00 + log(0.036) = 12.6

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The pH of rain water in northeastern United States on a particular day was 4.82. Find [H+].

pH = -log [H+]

[H+] = 10-pH

[H+] = 10-4.8

[H+] = 1.5*10-5 M

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Acid - Base Strengths

- Two factors influence acid strength:

1) Bond Polarity - The more polar the bond, the stronger the acid.- HCl (∆EN = 0.9) > > H2S (∆EN = 0.4)

2) Bond Strength - The weaker the bond, the stronger the acid.- HI >> HF. HI bond is weaker than that of HF bond.- Bond strength is the larger of the two factors.

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Strength of Oxyacids